On Tuesday, the New York Timesreported that there have been 34 percent fewer homicides in Chicago this year compared with last year—and that Mayor Rahm Emanuel and Police Superintendent Garry McCarthy are taking credit for the drop. The piece is balanced and thorough but hedges on saying whether or not the decline will last. While the Times might demur from stating the obvious, I won’t: The Chicago Police Department’s strategy here is not sustainable.

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To the extent that the decline in the murder rate is attributable to police tactics, we can credit McCarthy’s decision earlier this year to flood several crime-ridden “impact zones” with hundreds of cops pulling overtime duty. Saturating high-crime areas with more cops does drop crime rates in the short term. But the overtime money in Chicago is almost gone—as the Times reports, Chicago has already spent $32 million of the year’s $38 million overtime budget.

While Emanuel tells the Times that “the city will find a way to pay” for the increased police presence, it’s hard to see where the money will come from. The CPD’s total budget for 2013 is about $1.25 billion. Of that, about $1.2 billion is devoted to personnel costs, and most of that goes to salaries, so Emanuel won’t find any budget latitude there. The remaining $50 million mostly goes to contractors—phone services, waste disposal—and supplies. Emanuel could maybe move some money over from those budget lines, but that’s not much cash to play with. And besides, robbing Sgt. Peter to pay Det. Paul is not a sustainable strategy.

The fact is that you can’t pay extended overtime indefinitely. It saps the budget and it makes cops overworked and tired. Once the money runs out—and it will run out—crime in Chicago is going to rise, either in the impact zones that are no longer being flooded with cops, or in other areas where budgets have been reduced to pay for cops in the impact zones.

It’s also not clear that the impact zone strategy is what’s responsible for the decline. The drop in Chicago’s murder rate correlates to an unseasonably cold winter and spring this year. Plenty of evidence indicates that crime rates tend to rise and fall along with the temperature—last year, Chicago Magazine’s Whet Moser offered six possible explanations for why this might be. 2012 was Chicago’s warmest year in 142 years; when the temperature rises in Chicago this year, I expect the murder rate to rise as well. Instead of chewing through the CPD’s overtime budget, maybe the city would be better off just buying everyone in the impact zones some air conditioners.

Justin Peters is a writer for Slate. He is working on a book about Aaron Swartz, copyright, and the rise of “free culture.” Email him at justintrevett@fastmail.fm.